1. Field of the Invention
The present invention relates to work piece holders for use in conjunction with a surface measuring apparatus and, more particularly, toward work piece holders that allow a work piece to be moved relative to the surface measuring apparatus so as to reposition the work piece for access by the surface measuring apparatus.
2. Description of Related Art
Surface measuring apparatuses are well known devices that are useful for checking a surface profile of a work piece. Such surface measuring machines include a probe that is mounted to a head which, in turn, is mounted to an upright support. The head is movable vertically (z-axis) along the length of the upright support, and is pivotally or rotatably movable about a horizontal axis (parallel to the y-direction) through an angle (angle α). The probe is movable along the length of the head in an x-direction relatively toward and away from the work piece, and is also rotatable about an axis extending along the length of the probe (angle β). The upright support is movable horizontally in a y-direction, which is perpendicular to the x-direction. Such a surface measuring apparatus is sold by Mitutoyo Corporation as model number SV-3000 CNC.
One limitation with this known surface measuring machine is that the maximum translation of the probe in the x-direction, toward and away from the work piece, is limited, making it difficult to handle large work pieces. Increasing this range of motion by modifying the apparatus is possible, but very expensive, and not necessary in most surface measuring procedures. Another limitation with known machines is that manual re-calibration of the machine is normally necessary for each work piece, which is time consuming and labor intensive, and greatly reduces productivity of the machine.
Work piece holders are used to stably hold and reposition work pieces for a surface measuring operation. Such work piece holders tend to be customized to accommodate the particular work piece that is being measured, or to be specially adapted to move the work piece in a desirable fashion during the surface measuring operation. For example, it is known to use a rotary work piece holder, which is similar to a lathe, for holding a rotating a cam shaft during measuring of the cam shaft lobes and journals.
Other work piece holders permit the work piece to be moved in multiple planes so as to reposition the work piece during the surface measuring operation. For example, U.S. Pat. No. 6,671,973, which is assigned to Mitutoyo Corporation, teaches a work piece holder for a surface measuring machine. The work piece holder, which presumably is for relatively small work pieces, is disposed upon a table of the surface measuring machine. The work piece holder includes a y-axis table that is movable in an x-direction and slidable in a y-direction, a swivel table that is rotated in the x-y plane, and a stage, disposed on the swivel table, that is movable in a z-direction. In this way, the work piece holder disclosed in the '973 patent permits the work piece to be moved in two mutually perpendicular directions, and rotated about an axis extending in a third direction. Unfortunately, despite the improved work piece positioning capabilities afforded by the device taught in the '973 patent, portions of the work piece remain inaccessible due, in part, to limited travel of the probe in the x-direction.
U.S. Pat. No. 6,671,571, the entire disclosure of which is expressly incorporated herein by reference, teaches a method of using machining information, previously used to control machine tools, for subsequent work piece measurement to determine or evaluate a quality of the machining operation. The '571 patent provides a great improvement in work piece measuring as manual re-calibration is no longer required. Rather, with the method taught in the '571 patent, reference datum used during a machining operation can be taken off of a part support and subsequently used, by accessing data used during the machining operation, to permit the computer controlled surface measuring machine to automatically compare measured data with the machining data so as to determine whether the machining of the work piece has been properly performed. Naturally, in order to fully utilize the automated method taught by the '571 patent to its full advantage, access must be provided to each machined surface of the work piece. However, this is difficult or impossible with known work piece holders.
Accordingly, there exists a need in the art for a work piece holder that permits improved access to the held work piece so as to permit improved automatic or semi-automatic surface measuring. There further exists a need in the art for a work piece holder that is useful in conjunction with a surface measuring machine to automatically or semi-automatically permit measuring of the surface of a held work piece.